The states could do the same for segregation, too, barring blacks from certain schools, business, neighborhoods, etc.
But one wonders, in both cases, how long the federal government would allow such practices to continue.
An amendment to the U.S. Constitution, similar in wording to the 21st, would be the best, and fairest, approach to turning the regulation of drugs over to the states.
Legally, the issue already belongs to the states, unless and until an amendment to the US Constitution, similar in wording to the 18th, creates the necessary federal power.